Thymosin fraction 5, known, for example, from U.S. Pat. No. 4,082,737, is a potent immunopotentiating preparation and can act in lieu of the thymus gland to reconstitute immune functions in thymic deprived and/or immunodeprived individuals. Ongoing clinical trials with fraction 5 suggest that thymosin is effective in increasing T cell numbers and normalizing immune functions in children with thymic dependent primary immunodeficiency disease and can increase T cell numbers in immunodepressed cancer patients.
The first active peptide isolated and characterized from thymosin fraction 5 has been termed thymosin .alpha..sub.1. See, for example, U.S. Pat. No. 4,079,127 for a description of this peptide's isolation and characterization. Synthesis of thymosin .alpha..sub.1 by solution and solid phase synthesis techniques is described in U.S. Pat. No. 4,148,788. Additionally, the synthesis of thymosin .alpha..sub.1 by solution phase procedures is shown in U.S. Pat. No. 4,116,951. Thymosin .alpha..sub.1 has been found to be one or more orders of magnitude more active than fraction 5 in several in vitro and in vivo assay systems designed to measure T cell differentiation and function. Thymosin .alpha..sub.1 is currently in the clinic to determine its efficacy in the treatment of immunodeficiency diseases, immunodepressed cancer patients and in the prevention of opportunistic infections in immunosuppressed patients.
More recently, another related peptide designated thymosin .alpha..sub.11 was shown to share the biological activities of thymosin .alpha..sub.1. This peptide contained seven additional amino acid residues at the carboxy terminus when compared to thymosin .alpha..sub.1. A description of its isolation and characterization is given in U.S. patent application Ser. No. 532,418, filed Sept. 15, 1983, now abandoned, inventor B. Horecker.